Lasers that operate in a single spatial mode are employed in applications such as those that involve efficient coupling to a single-mode optical fiber and those in which a diffraction-limited beam is needed, e.g., in optical data storage, laser printing, etc. Semiconductor lasers capable of operating in a single spatial mode include buried heterostructure waveguide lasers.
A buried heterostructure waveguide laser includes a buried heterostructure waveguide made by fabricating a layer structure composed of layers of materials having different refractive indices sequentially deposited on a substrate. The buried heterostructure waveguide is characterized by a very large refractive index step (Δn˜0.1) between an active stripe and cladding layers located immediately next to the active stripe in the layer structure. The width of the heterostructure waveguide in the direction parallel to the major surface of the layer structure is typically made very small (˜1 μm) to minimize the threshold current (Ith<10 mA) of the laser. The buried heterostructure waveguide laser typically includes a pair of lateral confinement structures that define the width of the buried heterostructure waveguide.
A buried heterostructure waveguide laser can be easily fabricated of aluminum-free materials, such as indium gallium arsenide phosphide (InGaAsP), because lateral confinement structures can be formed by multiple regrowths in such materials without introducing interface states at the regrowth surfaces. However, when the active region of the laser is fabricated of materials that contain aluminum, such as aluminum gallium arsenide (AlGaAs), a simple regrowth process cannot be used. Accordingly, lasers whose active layer includes materials containing aluminum are typically configured as ridge waveguide lasers or include a laterally-oxidized structure to define the width of the waveguide.
Layer disordering is another option for defining the width of the waveguide of a buried heterostructure waveguide laser in which one or more of the materials of the active layer includes aluminum. The lateral confinement structures are composed of a semiconductor material formed by the layer disordering. Typically, a donor impurity, typically silicon (Si), is selectively diffused into the regions where the lateral confinement structures are to be located. The n-type impurity increases the concentration of vacancies in the group III lattice, which promotes intermixing of the group III elements. The intermixing occurs by the aluminum (Al) and gallium (Ga) atoms moving on the group III sublattice via the vacancies created by the donor impurity. The aluminum that diffuses from layers in which the aluminum fraction is higher into layers in which the aluminum fraction is lower tends to homogenize the aluminum fraction in the regions of the layers located in the lateral confinement structures.
The increased aluminum fraction in the lateral confinement structures increases the bandgap energy of the material of lateral confinement structures, and, as a result of the Kramers-Kronig relationship between band gap energy and refractive index, decreases the refractive index of this material. The semiconductor material formed by intermixing on the group III sublattice promoted by the diffusion of a donor impurity such as silicon provides a strong (Δn≧0.1) lateral optical confinement structure in the layer structure.
Intermixing of atoms on the group III sublattice is an effective technique for producing strong lateral confinement structures in AlGaAs-based index-guided lasers. Buried heterostructure waveguides using lateral confinement structures fabricated by intermixing of atoms on the group III sublattice have been used to make very high power single-mode lasers that operate in the wavelength range from 780 to 980 nm.
The optical fibers used in optical communication systems have their lowest losses in the wavelength range from about 1.2 μm to about 1.65 μm. The International Telecommunications Union has established an optical frequency standard grid covering the wavelength range from about 1.492 μm to about 1.612 μm. Optical communication systems operating in this wavelength range use lasers to generate the optical signals. Recently, quantum well heterostructures in which the semiconductor material of the quantum well layers is gallium arsenide nitride or indium gallium arsenide nitride ((In)GaAsN) or gallium arsenide antimonide (GaAsSb) and the semiconductor material of the barrier layers is aluminum gallium arsenide or gallium arsenide ((Al)GaAs) have been proposed for use in buried heterostructure waveguide lasers that generate light at wavelengths longer than 1.25 μm.
What is needed is a lateral confinement structure for use in long-wavelength buried heterostructure waveguide lasers having a quantum well heterostructure in which the semiconductor material of the quantum well layers is (In)GaAsN or GaAsSb and the semiconductor material of the barrier layers is (Al)GaAs.